The ca. 1.18–1.14 Ga A-type granites in the southwestern Yangtze Block, South China: New evidence for late Mesoproterozoic continental rifting

Abstract

The late Mesoproterozoic tectonic history of the Yangtze Block, South China in the context of the supercontinent cycles remains poorly understood due to the scarce preservation of geological records of this age. In this study, we report petrology, whole-rock geochemistry, zircon U-Pb ages and Hf-Nd isotopes for newly identified late Mesoproterozoic monzogranites from the southwestern Yangtze Block. SHRIMP and LA-ICP-MS zircon U-Pb dating results reveal that these monzogranites crystallized at ca. 1.18–1.14 Ga. All the studied samples possess typical geochemical signatures of A1-type granite, such as relatively high SiO2 (73.31–77.50 wt.%), Na2O + K2O (7.14–8.09 wt.%) and Zr + Nb + Ce + Y (362–445 ppm) contents, zircon saturation temperatures (825–851 °C), and low Y/Nb ratios (0.78–1.04). They have negative whole-rock εNd(t) values from −6.9 to −5.7 and zircon εHf(t) values from −7.4 to −2.5, with consistent Nd and Hf model ages of 2.4–2.1 Ga. The parental magmas for these monzogranites were generated by partial melting of the early Paleoproterozoic crustal materials at low pressure and high temperature. Taking into account of the coeval within-plate mafic magmatism in the southwestern Yangtze Block, these ca. 1.18–1.14 Ga A1-type monzogranites were most likely emplaced in a continental rift setting. Available data indicate continental rifting may have occurred extensively in the Yangtze Block at 1.2–1.1 Ga, followed by oceanic plate subduction starting at 1.1 Ga. We propose the Yangtze Block was an isolated plate during 1.2–1.1 Ga and started to drift toward the Rodinia supercontinent at ca. 1.1 Ga, favoring the proposal that a coherent supercontinent did not yet exist in the late Mesoproterozoic.